Propeller with annular connecting element interconnecting tips of blades

ABSTRACT

A propeller (10) including a hub (11), a plurality of blades (13) extending outwardly from the hub (11) and part annular connecting elements (17) which interconnect tips (16) of adjacent blades (13). The trailing edges (21) of the elements (17) extend from the trailing edge of one blade (13) at its tip to intersect the adjacent blade (13) at a relief point (23) on the high pressure side of the blade (13) between the leading and trailing edges of the blade (13). The leading edge (20) of the elements (17) extends from the leading edge of a blade (13) at its tip (16) to intersect the adjacent blade (13) at a relief point (25) on the low pressure side of the blade (13) intermediate the leading and trailing edges of the blade (13) at its tip (16).

This application has been filed under 35 U.S.C. 371 and claims foreignpriority benefits under 35 U.S.C. 119 of PCT/AU95/00826, filed Dec. 6,1995, which is based upon Australian patent, Serial#: PM 9879, filedDec. 6, 1994.

TECHNICAL FIELD

This invention relates to improvements to propellers and in particularto improved marine propellers.

BACKGROUND ART

Many different propeller constructions have been proposed in the pastand are presently available. Some propellers which have been proposedincorporate a ring or shroud which surrounds the propeller blades and isfixed thereto so as to be rotatable with the blades. The aim of suchshrouds is to direct energy rearwardly from the propeller, rather thanlosing energy as a result of centrifugal action. Such propellers havenot proved particularly effective and often have substantially decreasedefficiency compared to normal unshrouded propellers. In particular,excessive pressure can build up within the ring and furthermore, viscousdrag which occurs about the ring as it rotates builds up a rotationalboundary layer about the ring increasing the effective drag area of thepropeller.

One such propeller is disclosed in International Patent Application No.PCT/AU91/00582. The propeller described in that document includes anannular ring which joins the tips of the blades of the propeller andleaves free a region only on the high pressure side of the blades. Ithas been found in practice that this propeller has some disadvantages.

Other propellers have been provided with tip or end plates at the end ofthe propeller blades for distributing the vortices from the blades sothat minimum kinetic energy losses occur. Current tip and end platedesigns have had limited success.

DISCLOSURE OF THE INVENTION

The present invention aims to provide in one aspect an improvedpropeller, particularly suited to marine applications which functionsefficiently and which is safe in operation. The present invention alsoaims in a preferred aspect to provide a propeller which preventsexcessive vortex formations, particularly at the tips of the blades ofthe propeller. Other objects and advantages of the invention will becomeapparent from the following description.

The present invention thus provides in a first preferred aspect apropeller having a central hub, a plurality of blades spaced around saidhub and extending outwardly of said hub and having outer tips, andannular connecting means joining said tips of respective said blades,said annular connecting means joining adjacent said blades on theleading and trailing sides thereof respectively and leaving a region ofeach said blade at said tips on the leading and trailing sides free foroutward flow of liquid.

The annular connecting means may be in the form of part annularconnecting elements having leading and trailing edges, relative to thenormal forward direction of movement of the propeller, the annularconnecting elements extending around and lying on or substantially onthe surface of imaginary cylinder arranged concentrically with thepropeller hub. In one form, the trailing edge of each annular connectingelement may extend between the trailing edge of one blade at its tip anda first relief point on the pressure side or back face of the adjacentblade at the blade tip, the relief point being disposed intermediate theleading and trailing edges of the blade at the tip. The pressure side orback face of the blade is the side of the blade facing rearwardly withrespect to the direction of forward movement of a water craft with whichthe propeller is used. The leading edge of each connecting element mayextend from a second relief point on the negative pressure or suctionside of a blade intermediate the leading and trailing edges of the bladeat its tip and the leading edge of the adjacent blade at its blade tip.The negative pressure or suction side of the blade is the side of theblade facing forwardly relative to the direction of movement of thewater craft with which the propeller is used.

In one preferred form, the annular connecting elements may be ofsubstantially constant width in the axial direction and for this purposethe leading and trailing edges of the element are substantially parallelto each other. The annular elements between the respective blades may bearranged along a helix or helixes on the centred on the rotational axisof the propeller and thus are angled to the axis of the propeller. Thefirst and second relief points may be positioned directly opposite eachother on opposite sides of a blade at its tip. The relief points howevermay be positioned at any location between the leading and trailing edgesof the blade at the blade tip.

The elements joining the blade tips, however, may be in an alternativeconfiguration. For example, the trailing edges of the elements mayextend rearwardly beyond the trailing edges of the blades at the tip.Similarly, the leading edges of the blades may extend forwardly beyondthe leading edges of the blades at the blade tips.

Furthermore, whilst the leading and trailing edges of the elementspreferably straight or curved uninterrupted linear form, they may bestepped or provided with shoulders along their length.

Propellers according to the present invention may have a similarexternal diameter to the diameter of a conventional open propeller ormay be of larger or smaller diameter than an equivalent conventionalpropeller.

The pitch of the blades of the propeller may be constant along theirlength or may have a variable face pitch with pitch diminishing from theroot of the blade (at the hub) to the tip of the blade.

The propeller of the invention may have any number of blades rangingfrom two upwards, however, practically two to six blades prove mostefficient. The blades may be fixed to the hub or formed integrally withthe hub to extend outwardly therefrom.

In most outboard configurations the hub diameter ratio has a set meanwhich is enforced by the need to exit exhaust gases through the hub.Similar ratios may be employed in the propellers of the invention. Innon exhaust vent hubs however, the propeller may have a hub of smallerdiameter and mass as allowed by the additional structural integrityimparted to the blade hub connection by the support of the blades by theannular elements. The mean width ratio of the propellers fall withinknown parameters for conventional open and shrouded propellers.

In cross-section, the blades of the propeller may have lower or higherchord ratios than conventional propeller ratios. In some forms theblades may in cross-section have parallel curved faces whilst for higherrotational speeds, blades with a general wedge shaped cross-section withthe widest section being the trailing edge are advantageous.

In propellers according to the invention wherein the blades aresupported by the annular elements the blade thickness may be reducedbecause of the reduced need for cantilever strength due to the supportof the tips by the annular elements.

Blade contours can differ from most conventional open propellers and canbe parallel sided or varying in width from root to tip.

As with conventional propellers the skew or sweep of the blades fallswithin the general design rules, that is no skew or sweep for lowerrotational and surface speed propellers to increased skew or sweep forhigher surface and rotational speed.

A blade rake angle of zero degrees is suited to low speed (displacementvessel) operations whereas for higher speeds (planing vessels), it ispreferred to have a positive blade rake to minimize the drag effectscreated by the rotational boundary layers generated by the annularconnecting elements or tips due to viscous drag.

The annular connecting means of the invention serve to contain vorticeson either side of the blades and additionally reduce tip vortices. Thisminimizes viscous drag and allows rotational speeds similar to that ofconventional open propellers. The propellers are thus suitable foroutboard motor applications where maximum power and torque values areobtained at near maximum engine RPM. The annular elements are shaped toprovide minimum viscous drag as presented to the water flow and as theelements intersect the blades at relief points which allow outward flowof liquid, rotational boundary layers as encountered in current ringpropellers are reduced.

The relief point on the pressure side of the blade may be less than 0.5blade width at its intersection and attachment to the elements from thetrailing edge of the blade. By moving the relief point rearwardly, thatis towards the trailing edge of the blade, and thus increasing theamount of the blade tip encompassed by the element, the disciplineimposed on the water race is increased. This increases the thrustcapability of the device at lower speeds and is useful for high bollardpull applications.

The positioning of the relief points further defines the amount ofviscous drag penalty, and this becomes an important feature of thedevice when determining specific design criteria. As stated however therelief points may be positioned anywhere between the leading andtrailing edges of the blade depending upon the application.

The cross-sectioned shape of the annular elements varies with the dutyperformance required by the propeller as matched to the vessel. Theelements may have an external face along its length which is angled orparallel to the central axis. The elements may also have an externalfoil or ogival shape.

The internal profile of the elements may be of foiled shape, haveleading or trailing edge relief tapers or curved faces parallel orangled to the centre axis. The elements may also have parallel outer andinner faces which are angled to the central axis of the hub. The leadingedge of the elements may be rounded or tapered to a point with either orboth internal and external relief angles. The trailing edge may berounded, tapered, square or feathered to a point.

In one form, the elements may be of an aerofoil cross-section. Infurther forms the inner face of the elements may be planar or concave.

The hub may be parallel or tapered to the central axis and at thetrailing edge, the hub may exhibit a profile of constant cross sectionor be developed conically or flared outwardly in an alternative mannerso as to increase in diameter. The conical or flared development willusually commence at a point not greater than 20% of the hub length whenmeasured from the trailing edge. This conical or flared development willassist in extending the disciplined section of the water race avoidingpremature disintegration.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put intopractical effect, reference will now be made to accompanying drawingswhich illustrate a preferred embodiment of the invention and wherein:

FIG. 1 is a plan view of a propeller according to the present invention;

FIG. 2 is a side elevational view of the propeller of FIG. 1;

FIG. 3 is a perspective view illustrating portion of the shroud andassociated propeller blade of the propeller;

FIG. 4 is a sectional view of the propeller along line A--A of FIG. 2;and

FIG. 5 is a developed view showing the configuration of the bladejoining elements of the invention.

DETAILED DESCRIPTION OF MODES FOR CARRYING OUT THE INVENTION

Referring to the drawings, there is illustrated a propeller 10 accordingto the present invention including a central hub 11 of generallycylindrical form and including a splined sleeve 12 so as to enable thepropeller to be mounted to the splined driving shaft of a drive motor,for example an outboard motor. The hub 11, of course may be providedwith any means known in the art to enable it to be mounted to a driveshaft such as by means of a pin extending diametrically through the huband shaft.

Fixed to the hub 11 and extending outwardly therefrom are a plurality ofblades 13, in this instance three which may be either formed integrallywith the hub 11, for example by being cast therewith or secured to thehub 11 by welding or other means. In this embodiment and is moreapparent in FIG. 4, the blades 13 have a varying pitch from root to tip,and preferably curved leading and trailing edges 14 and 15 which taperto the tip 16 of the blade 13.

A plurality of part annular connecting elements 17 are provided betweenthe respective blades 13, the elements 17 being arranged in a direction,looking axially of the hub 11, on a common radius and concentricallywith the hub 11 and being fixed or joined to the outer ends or tips 16of the blades 13. The elements 17 may either be formed integrally withthe blades 13 or secured thereto for example by welding. In effect, theelements 17 extend around and lie on the surface of an imaginarycylinder centred on the axis of the hub 11 (shown in FIG. 2). The innerface 18 of the elements 17 in this embodiment is curved as is the outerface 19, the faces 18 and 19 thereby tapering towards the leading andtrailing ends of the propeller 10 in the manner shown in FIG. 4 to be ofan aerofoil type cross-section. The elements 17, however, may haveparallel or substantially parallel outer and inner faces 17 and 18 so asto be of constant cross section or be of other cross sectional form asreferred to above.

As shown more clearly in FIGS. 2 and 3 the elements 17 preferably have asubstantially constant width (W) relative to the axial direction of thehub 11 and thus the leading and trailing edges 20 and 21 respectivelyare substantially parallel. Additionally, the trailing edge 21 of eachelement 17 extends between the trailing edge 22 of one blade 13 at itstip 16 and a relief point 23 where it intersects and joins the nextblade 13 at its tip 16, the relief point 23 being arranged intermediatethe leading and railing edges 14 and 15 of the blade 13 on the highpressure side or face 24 thereof.

The leading edge 20 of each element 17 extends from a relief point 25 onthe low pressure side or face 26 of the blade 13 intermediate theleading and trailing edges thereof at the tip 16, to the trailing edge27 of the blade tip 16 of the next blade 13.

The elements 17 thus in their circumferential direction are inclined tothe axis of the hub 11 as shown more clearly in FIG. 5 and extendsubstantially along a helix or helixes centred on the axis of the hub11. The leading edge 20 is thus at an acute angle to the hub axis,whilst the trailing edge 21 is at an obtuse angle to the hub axis.

In the embodiment illustrated, the relief points 23 and 25 are directlyopposite each other on opposite sides of each blade 13. The reliefpoints 23 and 25 on either side of the blade 13, however, may be in anyposition between the leading and trailing edges of the blade 13 at itstip 16.

The hub 11 of the propeller 10 may include on its trailing side anoutwardly flared portion 26 which is of curved form in this embodimentbut which may be conical or outwardly divergent in any other manner. Theflared portion 26 provides for further guided movement of waterrearwardly of the propeller 10 upon rotation thereof.

Upon rotation, and on the high pressure side 24 of the blades 13, theregions 27 free of the elements 17 relieve build up of pressure withinthe elements 17 by permitting outward flow of water as indicated by thearrows in FIGS. 1 to 3. Additionally, water escaping outwardly throughthe region 27 hinders the development of vortices at the tips 16 of theblades 13 and confines the vortices about the elements 17.

On the low pressure side 26 of the blades 13, the elements 17 constrainthe water flow to concentrate the low pressure area to thereby increasesthrust. The elements 17 also serve to reduce tip vortices. Additionally,the regions 28 free of the elements 17 on the low pressure side of theblades 13 permit outward flow of water.

The elements 17 also serve, as well as an annular support to the blades13, as a safety device so that the tips 16 of the blades 13 are notexposed. Thus, damage to marine life will be substantially reduced andsimilarly the risks of damage to persons struck, inadvertently by such apropeller 10 will also be reduced. The annular elements 17, furthermoreform a nozzle about the axis of the propeller. Depending upon the angleof the elements 17 in relation to the axis of the propeller, theelements may effectively form a divergent or convergent nozzle about theblades 13.

The propellers of the invention may be formed of any suitable materialwith particular preferred materials being cast aluminium or mouldedplastics. The use of the elements 17 makes the propellers 10particularly suited for manufacture from plastics such as by aninjection moulding technique as the blades may be of thinnercross-section as the elements 17 enhance structural rigidity. Thispermits less material to be used thereby reducing cost of manufactureand increasing production efficiency. The elements 17, of course, mayalso extend beyond the blades 13 in an axial direction to the trailingand/or leading side of the blades as shown in dotted outline in FIGS. 2and 3, provided that a region on the trailing side is left open foroutward passage of water as described.

The width of the annular elements 17 in the axial direction depends uponthe application however generally the width is in the range of 0.1 to0.25 of the effective diameter of the annular connecting elements 17.Maximum thickness of the elements may typically be in the range of 0.015to 0.1 of the effective diameter of the elements 17.

Whilst the above has been given by way of illustrative embodiment of theinvention, all such modifications and variations thereto as would beapparent to persons skilled in the art are deemed to fall within thebroad scope and ambit of the invention as herein defined in the appendedclaims.

We claim:
 1. A propeller having a central hub, a plurality of bladesspaced around said hub and extending outwardly of said hub and havingouter tips, and annular connecting means joining said tips of respectivesaid blades, said connecting means joining adjacent said blades on theleading and trailing sides thereof respectively and leaving a region ofeach said blade at said tips on the leading and trailing sides free foroutward flow of liquid.
 2. A propeller according to claim 1 wherein saidannular connecting means comprises part annular connecting elementsconnecting adjacent said blades.
 3. A propeller according to claim 2wherein said hub has a central axis and wherein said annular connectingelements extend around and lie substantially on the surface of animaginary cylinder concentric with said hub.
 4. A propeller according toclaim 3 wherein said blades have leading and trailing edges relative totheir normal direction of rotation and wherein said annular connectingelements have leading and trailing edges relative to the normal forwarddirection of movement of said propeller.
 5. A propeller according toclaim 4 wherein said trailing edge of each said connecting elementextends between the trailing edge of one said blade at its tip and afirst relief point on the pressure side of the adjacent said blade, saidfirst relief point being disposed between said leading and trailing edgeof said adjacent blade at said tip.
 6. A propeller according to claim 5wherein said leading edge of each said connecting element extends from asecond relief point on the suction side of one blade intermediate theleading and trailing edges of said one blade at its tip and the leadingedge of an adjacent said blade at its tip.
 7. A propeller according toclaim 6 wherein said connecting elements have a substantially constantwidth in the axial direction.
 8. A propeller according to claim 6wherein said annular connecting elements lie along a helix or helixescentred on said central axis of said hub.
 9. A propeller according toclaim 6 wherein the respective said first and second relief points aredisposed directly opposite each other on opposite sides of each saidblade.
 10. A propeller according to claim 6 wherein said annularconnecting elements have in cross-section a substantially aerofoilconfiguration.